1. Field of the Invention
The invention relates generally to the recovery of hydrocarbon reserves from a subterranean formation and, more particularly, to using a horizontal circular wellbore to improve the recovery of oil from a plurality of producer wellbores drilled into a subterranean formation.
2. Brief Description of Prior Art
In the production of oil from subterranean formations, the oil may be produced initially by allowing the oil to flow, as a result of the oil-bearing formation's natural pressure, to the surface through wellbores extending from the surface into the subterranean oil-bearing formation, without the use of pumps or the like. After the formation pressure has dropped to a value less than that required to cause fluids to flow to the surface at a satisfactory rate, pumps, gas lifts, and other devices are used to move fluids from the formation to the surface. This phase of production, which is referred to as primary production, is often practiced with wellbores drilled in a "nine-spot" pattern 10 as shown in the plan view of FIG. 1, depicting the prior art. The "nine-spot" pattern 10 includes an array of eight producer wellbores 12, 14, 16, 18, 22, 24, 26, and 28 drilled into a subterranean formation (not shown in FIG. 1) to a form a perimeter of wellbores which surround a central wellbore 20. The arrows 30 indicate the direction of flow of oil from the formation into the wellbores during primary production.
After the oil flow from the formation has become insufficient to justify continued primary production using devices such as pumps and gas lifts to remove fluids from the formation directly, the primary production is discontinued and enhanced oil recovery processes are used. Enhanced recovery of the oil can be achieved by a variety of techniques which will vary widely depending upon the particular formation of interest. Three such techniques commonly used are water flooding, gas flooding, and combinations of water and gas flooding, referred to as "WAG" flooding.
In water flooding, water, such as brine or filtered seawater, is injected as a wave of fluid into the oil-bearing formation and pushed from a water injection wellbore toward an oil production well. In the nine-spot pattern 10, shown in FIG. 2, water is injected through the central wellbore 20 into the formation and pushed outwardly in the direction of the arrows 32 toward the perimeter wellbores 12, 14, 16, 18, 24, 26, and 28. Initially, oil and, subsequently, oil and injected water, are recovered from the production wells. Additional quantities of oil can be recovered from many formations by water flooding.
Gas flooding has also been used alone or in combination with water flooding to recover additional quantities of oil from formations. The gas typically comprises an oil miscible solvent such as hydrocarbons containing from one to about five carbon atoms, carbon dioxide, nitrogen, and mixtures thereof and is injected from an injection wellbore across the depth of the oil-bearing formation to form an injection wave of gas passing through the oil-bearing formation toward a production well. The gas may be single contact or multi-contact miscible with the oil, as well known to those skilled in the art. In the nine-spot pattern 10, shown in FIG. 2, gas is injected through the central wellbore 20 into the formation and pushed outwardly in the direction of the arrows 32 toward the perimeter wellbores 12, 14, 16, 22, 24, 26, and 28.
FIG. 3, an elevation view of the nine-spot pattern 10 taken along the line3--3 of FIG. 2, shows the flow patterns of the foregoing water flooding and as flooding. In FIG. 3, the oil-bearing formation is designated by the reference numeral 40, and includes an overburden 42, which formation and overburden are shown penetrated by the central wellbore 20 and the perimeter wellbores 18 and 22. In operation, water is injected via the wellbore 20 into the formation 40 and, because water is heavier than oil, the water tends to "slump" in the formation, particularly if the formation is thick and highly permeable with good vertical communication. As a result, the water flows downwardly and outwardly through a flow path 44 in the formation 40 toward the perimeter wellbores 18 and 22. Gas can be alternated with water in a WAG process and injected via the wellbore 20 into the formation 40 and, because the gas is generally lighter than the water and the oil in the formation, the gas tends to rise in the formation, particularly if the formation has good vertical communication. As a result, the gas flows upwardly and outwardly through a flow path 46 in the formation 40 toward the perimeter wellbores 18 and 22. Oil in the flow paths 44 and 46 will be swept into the perimeter wellbores 18 and 22, but maximum oil recovery from the formation 40 is not achieved.
Because the flow path 44 of water is downwardly, and the flow path 46 of gas is upwardly, a region 50 is formed between the flow paths 44 and 46 adjacent to the perimeter wellbores 18 and 22, as well as each of the other perimeter wellbores 12, 14, 16, 24, 26, and 28, and other areas of the formation, through which little or no injected water or injected gas flows. It can be appreciated that, as a result, a drawback with the foregoing water flooding and gas flooding techniques is that additional oil is not recovered in the regions 50. Additionally, a sub-optimal water sweep occurs in the upper flow path 46 and a sub-optimal gas sweep occurs in the lower flow path 44, which leaves recoverable oil in these areas also.
A further drawback with the prior art is that relatively high pressure must be used to inject water and gas from the injector wellbore 20 into the formation 40 so that, as the water and gas disperse toward each of the perimeter wellbores, the pressure will not be dissipated below the pressure necessary to sweep oil to each of the perimeter wellbores.
Therefore, what is needed is a method and system for recovering oil in the flow paths 44 and 46 and in the region 50 which is not fully recovered by conventional water flooding, gas flooding, or WAG processes.